Use of uninitialized value $ _ in pattern match (m //) at cursor.pl line 5

I have this Perl script:

#!/usr/bin/perl
use strict;
use warnings;
while () {
    if (/^PARSING IN CURSOR/../END OF STMT/) {
        if (/^PARSING IN CURSOR/) {
            s/^PARSING IN CURSOR \#//;
            s/ [a-z]+=/!/g;
            s/\n$/!/;
            $_="$.!$_";
        }
        unless (/^END OF STMT/) {
            print;
        }
    }
}

and it shows me the message:

  

Use of uninitialized value $ _ in pattern match (m //) at cursor.pl line 5